With increasing restrictions on the use of landfills for waste disposal, incinerators have received greater acceptance as a means for reducing the volume of waste materials in addition to generating heat and/or power. In general, these incinerators are operated at elevated temperatures and produce combustion gases containing various pollutants which are usually reduced to acceptable levels before exiting through the stack to the atmosphere.
The levels and type of pollutants in the combustion gases prior to treatment and removal through the stack may vary depending on the combustion temperatures, type of fuels or waste materials being consumed, residence time of the waste materials in the incinerator, amount of air admitted to the combustion zone, and the procedures for adding the fuel to the incinerator. In some instances, higher temperatures in the order of 1000.degree.-1200.degree. C. are used for more complete combustion to reduce the levels of organics and partially oxidized organics in the combustion gases.
The formation of halogenated organics, free halogens and halogen acids are particularly a problem since many are toxic and/or corrosive. Examples of these substances are those containing chlorine, which are represented by the polychlorinated biphenyls (PCB), dioxins, chlorine gas and hydrogen chloride. At the higher temperatures, the combustion process can favor the formation of substantial amounts of chlorine and hydrogen chloride. With available hydrogen in the combustion process, chlorine is converted to hydrogen chloride which may be removed by the use of sodium carbonate, lime or a water spray. Removal of the hydrogen chloride in this manner may be accomplished with a scrubber.
However, both the toxic and corrosive nature of these halogen containing materials and their conversion to less objectionable products frequently involves problems. Combustion conditions and waste feed material may vary and cause changes in the amounts and type of the materials that are produced. Both halogens and halogen acids are corrosive and toxic and are not removed completely by the scrubbing operation. In addition, at lower combustion temperatures below about 700.degree. C., halogen and halogen acids may react with trace amounts of organics remaining in the combustion gases to form toxic halogenated organics.
Accordingly, one object of this invention is a process for reducing pollutants in combustion gases. A second object of the invention is a process for reducing the amounts of halogens, halogen acids, and halogenated organics in combustion gases. An additional object of the invention is a process for reducing pollutants and particularly halogen-containing pollutants in combustion gases without adding treatment chemicals to the combustion gases. A further object of the invention is a process for reducing pollutants and particularly the halogen-containing pollutants with the use of equipment that does not result in a waste water stream or any other residual that requires chemical or other treatment for neutralization or removal of pollutants. A further object of the invention is a process for reducing pollutants and particularly halogen-containing pollutants in such a manner as to recover halogens for resale or other useful purposes. These and other objects of the invention will become apparent from the following detailed description.